1. Field of Invention
This invention relates to the field of electrographic printers, specifically to a treatment for liquid toners used in such printers.
2. Prior-Art
Toner for Electrographic Printers
Electrographic printers are manufactured and sold by Xerox ColorgrafX Systems, Inc., 5853 Rue Ferrari, San Jose, Calif. 95138 U.S.A., and others. These printers typically comprise a supply roll of electrographic medium (typically a specialized paper), one or more electrographic writing heads, one or more "developing stations," a drive roller for moving the medium, and a take-up roller for spooling the medium after it is printed. The writing head deposits an electrical charge image on the medium, and the developing station applies liquid "toner" to the medium. Colored particles in the toner adhere to the charge pattern corresponding to the image to make the image visible. The toner air-dries, evaporating a solvent liquid to permanently adhere the particles to the medium.
Electrographic toner mainly comprises a slurry of electrically charged, colored, colloidal (sub-micron)-sized particles in a solvent vehicle. Other components of toner are also suspended and dissolved in the liquid vehicle. These ensure maintenance of the proper level of electrical conductivity in the toner, cause proper adhesion of the toner particles to the receiving medium, and provide for long toner life. Such toner is manufactured by Hilord Chemical Corporation, 70 Engineers Road, Hauppauge, N.Y. 11588 U.S.A. and others.
During printing, electrographic printers recirculate liquid toner from a reservoir where it is stored, through a "developing station," and back to the reservoir. At the developing station, the surface of the print medium is flooded with toner. Toner particles adhere to the medium in places where electrical charges have been deposited. They do not adhere elsewhere. The result is a visible image of the original, invisible electrical charge image.
As mentioned supra, the diameter of toner particles is typically less than one micron and other agents which are both suspended and dissolved in the toner vehicle also contribute to the toner's electrical, mechanical, and optical properties. This mixture of particles and complex liquid vehicle must be maintained in an homogeneous state in order for high-quality, consistent prints to be obtained. Gravitational forces cause suspended particles to settle to the bottom of their container. Because of their small size, the toner particles settle very slowly, typically in a few tens of hours. The toner particles are typically not adequately stirred by the pumping mechanisms in electrographic printers. This results in stratification of the toner in its container or reservoir. At the bottom of the reservoir is a slurry containing more particles than desired. These particles displace the chemical conductivity-control agents mentioned supra. The remaining toner liquid contains too few particles and has disproportionately high electrical conductivity. This non-homogeneity of the toner in the reservoir results in the printing of poor quality images.
It is mandatory that the concentration of charged, colored particles and other dissolved and suspended chemical conductivity-control agents in the toner remain constant during printing. If the concentration changes during the printing of an image, the optical density or saturation of the image will also change unpredictably, resulting in a degraded image. Settling of the toner cannot be allowed since this causes fewer toner particles to be recirculated and hence available for "developing" the image. A faint image, or one which changes from dark to faint during printing, can result.
Recirculation of the toner in the printer is generally inadequate to ensure that all the particles will remain suspended and that the slurry will be homogeneous. In some cases, the toner agglomerates at the bottom of the reservoir and cannot be dislodged, except by extraordinary means. These means include mechanically moving and shaking the toner reservoir to agitate the contents, periodically stirring the toner with a stick, and the like. None of these is adequate to ensure that the toner will remain in an homogeneous state for an indefinite period.
It has been known to stir various fluids by application of heat to cause convection currents in the fluid. For example, Hoisington, in U.S. Pat. No. 4,814,786 (1989) teaches the convective stirring of a non-electrographic solution, a hot-melt ink. However Hoisington's heaters must serve two distinct purposes. Heat is provided to raise the temperature of the ink to 120 deg. Celsius (248 deg. F) in two different vessels order to melt it. The same heat is applied in an asymmetrical fashion in both vessels additionally to provide convective stirring. The vessels are connected by a conduit which must be heated when it is desired to pump molten ink from one vessel to the other. Hoisington's printer will not work at all if his heaters are OFF since the ink becomes solid. Present electrographic printers, on the other hand, will work without proper stirring of the toner, but, as stated, image quality is degraded. Hoisington's ink typically comprises a simple suspension of pigment particles in a vehicle which changes state from solid to liquid and then from liquid to solid by phase changes during use. Liquid toner remains liquid and additionally contains a complex mixture of both dissolved and suspended chemical conductivity-regulating agents, particulates and plastic resins, described supra, all of which must be fully dispersed in the toner. Toner becomes so lid on the receiving me dium through evaporation of its liquid vehicle. No phase change is required. Hoisington's ink reservoirs are sufficiently small that only a single heat contact point is required to both melt and convect the molten ink. Electrographic printer toner reservoirs are sufficiently large that a heat source like Hoisington's will not be able to maintain adequate homogeneity by convective stirring.
Thus present electrographic printers suffer loss of print quality due to non-homogeneity of their toners. Prior-art stirring methods and apparatus do not render the toners homogeneous: hence print quality is variable, and thus degraded.